The present invention relates to improvements in machines for manufacturing in large quantities metal and alloy powders in the form of small diameter spheres the shape and grain size of which must be controlled within narrow limits. There is in fact a market for such powders which are employed particularly for their low specific surface since the shape of a sphere is that which presents the lowest surface area as compared with its volume, but also for their propensity to move across a plane surface at constant acceleration under the influence of a force of given magnitude, whatever the direction of this force in the plane of motion.
A method of manufacturing such metal powders with predetermined shape and dimensions by spraying from a rotating electrode is described, for example, in U.S. Pat. No. 3,099,041. In accordance with this method an electrode which is cylindrical or nearly a cylinder of revolution, made of the metal or alloy from which it is required to make the powder, and which will hereafter be called the rotating electrode, is set in rapid rotation about its longitudinal axis. The rotating electrode is then progressively melted, the plane of fusion being one of the plane surfaces bounding the cylinder perpendicularly to its longitudinal axis. The fusion may be obtained by striking an electric arc between this surface of the rotating electrode and an electrode of refractory metal formed in accordance with one of the methods known to those skilled in the art and which will hereafter be called the non-consumable electrode. The centrifugal force created by the rotation of the rotating electrode causes spraying of the molten metal in the form of droplets, which droplets solidify and cool during the course of their free trajectory. The process of formation of the droplets is well known and relationship between the size of the powder grains and various operational parameters has been defined, for example, by a law of the following type: ##EQU1## where D IS THE MEAN DIAMETER OF THE GRAINS OF POWDER IN MICRONS,
k is a constant, PA1 N is the speed of rotation of the rotating electrode in r.p.m., PA1 .gamma. is the surface tension of the liquid metal from which it is required to make the powder in erg/cm.sup.2, PA1 D is the diameter of the rotating electrode in cm., PA1 e.sub.L the density of the liquid metal in g/cm.sup.3.
In general a spraying device of this kind is located entirely within an enclosure which isolates it from the outer atmosphere and which enables it to be kept in an atmosphere which is neutral with respect to the metal or alloy to be sprayed, so as to avoid contamination of the powder by reactive gases such as oxygen and nitrogen. Putting the method into effect by means of the device that we have just described enables spherical powders of elemental analysis and controlled grain-sizes to be obtained from very diverse metals and alloys such as steels, alloys of nickel, cobalt, titanium, beryllium, niobium, molybdenum, tungsten, etc.
Known apparatus employing the method which we have just described are designed so that with fusion of the rotating electrode the distance between it and the non-consumable electrode remains substantially constant. An equidistance of this kind may be obtained by providing one of the two electrodes with a movement in translation parallel with the longitudinal axis of the rotating electrode. In one particular arrangement the non-consummable electrode is fixed and the rotating electrode is capable of being displace along its longitudinal axis. In another arrangement the rotating electrode is not capable of being displaced in translation, but the non-consumable electrode is displaced parallel with the longitudinal axis of the rotating electrode.
In all the know apparatus the non-consumable electrode is centred on the longitudinal axis of the rotating electrode or set slightly offcentre, but remains fixed in the plane perpendicular to the longitudinal axis of the rotating electrode. Such an arrangement limits the diameter of the rotating electrodes to a few centimeters at maximum (5 to 7 cm for a steel, for example). Indeed the zone which is brought to fusion point by the electric arc is limited to a circle of a few centimeters in diameter at the maximum. The liquid metal formed in this zone and carried away by the centrifugal force solidifies again before reaching the edge of the front surface of the consumable electrode if the diameter of the latter is excessive. In such a case the molten metal cannot be ejected in the form of droplets and one observes simply a hollowing of the central zone of the rotating electrode and a swelling of the peripheral zone.
In the simplest version of an apparatus operating in accordance with the above method the device for location of the rotating electrode is inside the airtight enclosure. The length of the rotating electrode is therefore limited by the dimension of the enclosure along the longitudinal axis of the rotating electrode but also by the disadvantages resulting from the overhang or free length of an electrode of rather small diameter rotating at high speed about its own axis, which speed is the higher the smaller the diameter of the electrode, as shown by the equation given above. Thus in order to obtain powders of 100.mu. from a rotating electrode of steel 65mm in diameter it is necessary to make it rotate at 10,000 r.p.m., and in order to avoid perceptible bending of the rotating electrode under the effect of the centrifugal force its length in overhang or free length is limited to 250mm which represents a consumable mass of a maximum of 6.5 kg. Hence the productivity of such an apparatus is low because of the necessity for frequently changing the rotating electrodes. Moreover the portion of the rotating electrode kept for attaching it to the driving device is not transformed into powder and represents a considerable proportion of the whole of the electrode. Finally the manufacture of the electrodes represents a considerable expense which is not paid off by a sufficient quantity of powder being manufactured.
With a view to alleviating the disadvantages resulting from the low unit mass of the rotating electrodes, two methods in particular have been proposed.
The first method, described in U.S. Pat. No. 3,802,816, consists in locating the driving system for the rotating electrode outside the airtight enclosure and making this system movable along the longitudinal axis of the rotating electrode. An airtight passage through the wall of the enclosure enables the rotating electrode to pass inside and its overhang or free length to be limited. Displacement of the driving system makes the electrode slide in the airtight passage in order to compensate for its progressive wear. The overhang or free length of the rotating electrode remains constant as it is sprayed away and limited to the length of the portion lying inside the enclosure. A method of this kind enables the length of the rotating electrodes to be substantially increased but this increase remains limited to three or four times that of the rotating electrodes employed in the simplest machines if the driving means is not to be excessively complicated. Again, it is necessary for the rotating electrode, which slides into the enclosure by way of a seal, to have a shape very nearly that of a perfect cylinder, which necessitates a particularly costly operation of machining by grinding.
The second method, described in U.S. Pat. No. 3,784,656 consists in employing as the rotating electrode a wire of great length. In order to communicate a sufficiently high centrifugal force to the molten metal the wire advances inside a rotating cylinder through a channel of section slightly greater than that of the wire, entering along the axis of rotation of the cylinder and leaving along an axis intersecting the axis of rotation at a large angle with respect to the latter so that the outlet orifice is offcentred with respect to the axis of rotation of the driving cylinder. The rotating system is thus equivalent to a rod of diameter much greater than that of the wire. The main disadvantage of this second method is that it requires the conversion of the metal or alloy to be sprayed into wire, which is in any case costly and is impossible for a number of metals or alloys the plasticity of which is insufficient to lend itself to known methods of wiredrawing.
In other known modifications of the above basic principle, the non-consumable electrode causing local fusion of the rotating electrode is replaced by other known means enabling the rotating electrode to be heated locally up to fusion point. Such known means may be a plasma torch or an electron gun which have been employed to enable fusion of materials which are poor conductors of electricity or the fusion temperature of which is particularly high. The apparatus using such modifications is designed on the same principles as those which we have described above and display the same disadvantages connected with limitation of weight, dimensions and form of the rotating electrodes.